Temperature measuring devices using materials that melt at different temperatures are known. Such devices have been used for the most part as disposable clinical thermometers. For example, one type of such prior art temperature measuring device, as described in U.S. Pat. No. 3,859,856, uses a backing member and several layers of chemicals disposed in compartments on the backing member. The layers of chemicals consist of first and third layers normally separated by a second layer in each compartment. The chemicals forming the second layer have characteristics for melting at a different known temperature for each compartment. In response to the melting of the chemicals in the second layer, the chemicals in the first and third layers mix and react to form a particular color. The backing member is provided with indicia adjacent the respective compartments representative of the melting temperature of the chemical in the second layer of the associate compartment. With these devices, the sensed temperatures ascertained by reading the highest number of the indicia corresponding to the compartments having the particular color.
These prior art devices have been used primarily with human beings to measure body temperatures which generally vary within a range of 95.degree. Fahrenheit and 105.degree. Fahrenheit.
The present invention provides a simple temperature sensor which is used to measure the electric current flowing through a conductor by measuring the temperature of the conductor. It is well known that due to its inherent resistance, a conductor carrying an electric current increases in temperature as the current increases. The instrument of the invention has a feature in that it is inherently simple in its construction and may be manufactured economically and sold at a relatively low price.
The embodiment of the invention to be described is one in which a series of compartments are provided along a current-carrying conductor, with each compartment being filled with a solid material, such as wax, that melts at different temperatures for different compartments. The material in each compartment is normally solid. However, when the temperature of the conductor which varies with the current flowing through the conductor reaches a particular level, the wax in one of the compartments melts, and becomes transparent. This enables the inner wall of the particular compartment to become visible, and to display a particular color. Then, for different currents, the wax in different compartments melts, so the back walls are displayed. The instrument may be calibrated in amperes when used to measure currents for different loads when the voltage is constant; or it may be calibrated in volts when the load is constant and the voltage changes.
The instrument of the invention has widespread application for both home and industrial use, for providing a simple means for indicating the temperature of a current carrying conductor, to provide an indication of the current flowing through the conductor, particularly to indicate when the current is approaching or has exceeded a danger level.